I find the best strategy is to take out as many enemies as you can between crates. But remember, the point of the game is to collect crates, not kill enemies. Don't get hung up because you like a gun, it'll come back eventually. Other than that, keep practicing and you'll learn how to time things and plan things mid flight.

After you've unlocked some of the better guns it gets easier. If you can get to the second level, it's a better one to learn on do to the platform in the middle that is usually a safe zone. Really you just have to pratice a lot. Clear the screen of as many enemies as you can, dash for the crate, return to a safe zone, rinse and repeat. And be careful about letting enemies fall into the fire and becoming angry, they've ruined games of mine way to many times:(

I reccomend getting unlocking better guns. You can unlock the new guns from the number of crates you gain. Don't worry. The game auto-saves each time you die so you don't have to collect a number of crates in one playthrough.